In a process for manufacturing a semiconductor element used in a magnetic random access memory (MRAM), a magneto-resistance effect type head, or the like, it is necessary to perform heat treatment of an object to be treated, in which a large number of 12-inch silicon wafers are stacked, while applying a high magnetic field equal to or larger than 500 gausses to the object to be treated. Therefore, it is necessary to secure a large space as a heat treatment space. Since the high magnetic field has to be generated in the large space, a superconducting magnet is used as a generation source of the high magnetic field (e.g., Japanese Unexamined Patent Publication No. 2013-138058).
Since the high magnetic field is generated in the large space, when a magnetic shield is not applied, a magnetic field from the superconducting magnet leaks to a wide range. An in-magnetic field heat treatment apparatus described in Japanese Unexamined Patent Publication No. 2013-138058 is set in a clean room. Therefore, a footprint is desirably small. The SEMI standard, which is a standard of a semiconductor manufacturing apparatus, provides that a fringe field in a position 1 m or more apart from the outer wall of the apparatus should be reduced to 5 gausses or less.
As a method of applying the magnetic shield, there are a method by an active shield and a method by a passive shield. The method by the active shield is described in, for example, paragraph 0019 of the specification and FIG. 3 of Japanese Unexamined Patent Publication No. 2013-137131. This method is a method of disposing, on the outer side of a superconducting magnet, a shield coil that is made of a superconductive conductor and generates an inverted magnetic field. The method by the passive shield is described in, for example, FIG. 1 of Japanese Examined Patent Publication No. H03-57776. This method is a method of disposing a magnetic material such as iron on the outer side of a cold insulation container (a cryostat) of a superconducting magnet.
The active shield method has an advantage that the superconducting magnet (a device) is not heavy because the magnetic material such as iron is not used. However, there is a concern that, when quenching occurs, a balance between generated magnetic fields of the shield coil and a coil on the inner side of the shield coil collapses and, therefore, a fringe field increases. On the other hand, in the passive shield method, there is no concern about the increase in the fringe field due to the quenching. However, since many magnetic materials such as iron are used as the magnetic shield, the device is increased in weight.